Update build.yml

.github/workflows/build.yml

@@ -12,5 +12,7 @@ jobs:
           ./elan-init -y --default-toolchain none
           echo "$HOME/.elan/bin" >> $GITHUB_PATH
       - uses: actions/checkout@v2
+      - name: get cache
+        run: lake exe cache get
       - name: build game
         run: env LEAN_ABORT_ON_PANIC=1 lake build